Research On Key Technologies Of DCO-OFDM In Broadband Visible Light Communication

In recent years,the development of the semiconductor industry makes the cost of light-emitting diode(LED)decrease continually,and LED has the advantages of small size,long service life,and high luminous efficiency,which makes LED gradually replace the fluorescent lamp and incandescent lamp as the main light source of lighting facilities.Therefore,visible light communication(VLC)technology,which uses LED to realize the dual functions of lighting and communication,has received more and more attention.The optical orthogonal frequency division multiplexing(O-OFDM)tech-nology is usually used in broadband VLC systems to resist the inter-symbol interference(ISI)caused by non-ideal channel characteristics,where the direct-current-biased optical OFDM(DCO-OFDM)is simple to implement and has high spectral efficiency,and it has been widely adopted in practical VLC systems.This thesis will make in-depth research on the key technologies of VLC DCO-OFDM systems.Firstly,this thesis introduces an overview of the VLC systems.This thesis first introduces the link structure and system noise of the VLC systems,and compares the main characteristics of the VLC system with the conventional radio frequency(RF)communication system.Subsequently,this thesis introduces the composition and characteristics of electro-optical conversion devices and pho-toelectric conversion devices used in VLC systems,and compares the performance of different types of optoelectronic devices.At last,this thesis discusses the characteristics of the VLC channel.Additionally,this thesis studies the optimal signal shaping design problem of the VLC DCO-OFDM system with single-sided clipping.This thesis first explains the cause of single-sided clipping is the intensity modulation with direct detection method adopted in VLC systems,and models the single-sided clipping process.Subsequently,this thesis transforms the optimal signal shaping design problem into the optimization problem of DC bias and subcarrier power,and proposes optimal signal shaping schemes in flat and dispersive channels respectively.In the end,the optimality of the proposed signal shaping schemes under different channel types is verified by simulations.Furthermore,this thesis investigates the optimal signal shaping design problem of the VLC DCO-OFDM system with a more general double-sided clipping case.This thesis first explains the cause of extra upside clipping is the limited dynamic range of LED,and models the double-sided clipping process.Then,this thesis transforms the optimal signal shaping design problem into the optimization problem,and decomposes it into two sub-problems about DC bias and subcarrier power to solve.Sub-sequently,the impact of upside clipping is analyzed by combining the optimal signal shaping schemes obtained in single-sided and double-sided clipping cases respectively.In the end,the optimality of the proposed signal shaping schemes and the related conclusions are verified by simulations.Finally,this thesis investigates the suppression of the peak-to-average power ratio(PAPR)of VLC DCO-OFDM systems.This thesis first gives the definition of PAPR,and deduces the statistical characteristics of the PAPR theoretically.Subsequently,this thesis introduces the basic principles and implementation steps of several conventional PAPR suppression algorithms detailedly,and analyzes the PAPR suppression performance of these conventional PAPR suppression algorithms in VLC DCO-OFDM systems by simulations.At last,this thesis proposes a PAPR suppression algorithm based on convex optimization.By designing an appropriate frequency-domain precoding matrix in the VLC DCO-OFDM systems using LED arrays,this algorithm can achieve better PAPR suppression perfor-mance than conventional algorithms on the premise of ensuring the signal quality at the receiving end.